According to the context the term that adequately completes the fragment is Herd immunity.
What is herd immunity?
Herd immunity is a term to refer to a biostatistical phenomenon that originates in a population when most of it has become immune to a disease (due to previous infection or because it has been vaccinated) and the epidemiological chain is interrupted, causing an indirect form of protection against a disease by preventing unimmunized individuals from becoming infected.
According to the above, the appropriate term to complete the fragment would be herd immunity.
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